First, if you have Google 2-step verification enabled (surely you do?), you need to add a new app password - otherwise turn on access for less secure apps.
Then sending the email is as simple as:
curl --ssl-reqd \
--url 'smtps://smtp.gmail.com:465' \
--user '[email protected]:password' \
--mail-from '[email protected]' \
--mail-rcpt '[email protected]' \
--upload-file mail.txt
Where email.txt can be something as basic as:
From: "User Name" <[email protected]>
To: "John Smith" <[email protected]>
Subject: This is a test
Hi John,
I’m sending this mail with curl thru my gmail account.
Bye!
or fully-fledged multipart text+html email:
MIME-Version: 1.0
Subject: This is a test
From: Maciej Zgadzaj <[email protected]>
To: Maciej Zgadzaj <[email protected]>
Content-Type: multipart/alternative; boundary="00000000000067dfe705d9d0ff7a"
--00000000000067dfe705d9d0ff7a
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Hi John,
I'm sending this mail with curl thru my gmail account.
Bye!
--00000000000067dfe705d9d0ff7a
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p>Hi John,</p>
<p>I'm sending this mail with curl thru my gmail account.</p>
<p>Bye!</p>
</body>
</html>
--00000000000067dfe705d9d0ff7a--
You can build your own, or just copy-paste the source of any received email (in Gmail it is under More > Show original), just stripping out any unnecessary headers.
Based on this StackOverflow (obviously!) response.